In general, authentication is the act of confirming the identity of a person such as a computer user. A simple approach for a computer to authenticate a user is for the computer to prompt the user for a username and password.
Multi-factor computerized authentication involves delivery of more than one authentication factor by the user. One factor may be something that the user knows such as a password. Another factor may be something the user has such as a keycard. A third factor may be something the user is such as a scan of some part of the user's body such as a fingerprint scan or an eye scan.
The latter two require deployment of additional devices at the user's side. To deliver a fingerprint scan to a computer, the user typically runs a finger over a fingerprint scanner which reads the user's fingerprint from the user's finger. Similarly, to deliver an eye scan to a computer, the user typically looks into an eye scanner which captures an image of the user's iris or retina.